1. Field of the Invention
The present invention is broadly concerned with new superconducting metallic oxides exhibiting high T.sub.c values on the order of 100.degree. K. and above, coupled with very high calculated J.sub.c critical current values of at least about 10.sup.4 amperes/cm.sup.2. More particularly, in one aspect the invention is concerned with such oxides defined by the general formula: EQU (V.sub.1-x M.sub.x).sub.i (A.sub.1-y M.sub.y).sub.j Q.sub.k Cu.sub.m O.sub.r.+-.t I.
where M is respectively taken from the group consisting of bismuth, lead or antimony, A is calcium, sodium or potassium, Q is different than A and is either strontium, barium or calcium, x and t range between 0 and less than 1, i is either 2 or 3, j is either 0, 1 or 2, k is 2 or 3, m is either 1, 2 or 3, and r is an integer, typically ranging from 8-12. In another aspect, the invention comprehends vanadium-containing oxides of the general formula EQU Bi.sub.2-p-q V.sub.p Pb.sub.q Sr.sub.2 Ca.sub.2 Cu.sub.3 O.sub.10.+-.tII.
where p and q each independently range up to about 0.7 and t ranges between 0 and less than 1. Superconducting oxides in accordance with the invention exhibit their high T.sub.c and J.sub.c values when the oxides are bulk sintered with an essentially random crystal orientation, so that the oxides are eminently suited for practical applications as high current-carrying conductors.
2. Description of the Prior Art
Superconductivity refers to that special state of a material where its resistance to electrical current flow suddenly and completely disappears when its temperature is lowered. Below this onset or critical temperature T.sub.c, a characteristic of the material, the electrical resistance does not merely drop to a low level but it vanishes entirely. Only a very limited list of materials exhibit such a state. The discovery of the first superconductor occurred in 1911. Heike Kammerlingh Onnes discovered that Mercury lost all detectable resistance at a temperature just 4.degree. above absolute zero.
A superconductor also exhibits perfect diamagnetism below its critical temperature, i.e., it expels all magnetic field lines from its interior by producing an opposing magnetic field from a current flowing on its surface. As a consequence of the perfect diamagnetism of superconductors, they can be used to produce magnetic levitation as envisioned in high speed transport systems of the future, where magnetic repulsion is used to counter gravity. The perfect diamagnetism property of superconductors is called the Meissner effect after its discoverer.
Superconductivity is the only large scale quantum phenomenon involving charges found in solid materials. The current-carrying electrons in the superconductor behave as if they were part of a monumentally large single molecule the size of the entire specimen of the material. The macroscopic quantum nature of superconductors makes them useful in measuring magnetic field quantities to high precision or facilitates the measurement of such quantities so small as to be heretofore unmeasurable.
Hence, all three aspects of superconductors give promise of exciting new technologies or improvements in old technologies. However despite the tremendous potential of superconductors, formidable technical problems must be overcome if such materials are to achieve practical commercial application. For example, until very recently, all known superconducting materials attained their superconducting state only at very low (cryogenic) temperatures on the order of 4.degree.-20.degree. K. Such low temperatures had to be reached by evaporating liquid helium, the only substance that remains liquid down to temperatures approaching absolute zero. The few sources of helium in nature and its expensive processing make it a very costly cryogenic fluid.
In recent years, a plethora of new superconducting oxides have been announced by researchers around the world. While these new materials have relatively high critical temperatures on the order of 80.degree.-130.degree. K., they are plagued by a number of intractable problems. For example, certain of these prior materials, while they have high T.sub.c values, have very low (e.g., 1 ampere/cm.sup.2 current density values, particularly when the materials are bulk sintered and therefore have random crystal orientation therein. Such prior materials may exhibit higher current densities, but only when formed as oriented epitaxal films on substrates. Obviously, such materials, while they exhibit superconducting properties, are totally impractical for use in most commercial applications. Finally, many of these prior superconductors are extremely brittle and frangible, which again effectively precludes their use as commercial-scale electrical conductors for example.
Accordingly, while there is recent intense interest in superconducting materials, presently available oxides of this character have one or more serious deficiencies which render them useless in commercial applications.
Certain vanadium-containing oxides are described in Che et al., Journal of Materials Science, 24, p. 1725-1728, May, 1989. The authors of this article disclose that vanadium, when substituted into bismuth precursors, generally depresses T.sub.c values, and that no superconductivity obtains when the substitution exceeds 0.5. In FIG. 4 of this article, a maximum T.sub.c of slightly over 80.degree. K. is described for a vanadium-containing oxide.
The vanadium oxide needed in this article is V.sub.2 O.sub.5 (Vanadium pentoxide). According to the chemical composition formulae I and II in the article, vandium is substituted for trivalent bismuth. Thus it is highly possible that the negative results obtained in this article were entirely due to using a less advantageous vanadium oxide in the preparation, together with improper heating and sintering conditions.